In general, photonic crystals have a periodic structure in which materials with different refractive indices or dielectric constants are repetitively arranged at regular intervals equivalent to the wavelength of light. Such photonic crystals have a photonic bandgap region in which light in a specific wavelength range are all reflected, and thus the light in the specific wavelength range does not exist inside the photonic crystals.
Particularly, the photonic bandgap of the photonic crystal is determined by a difference in refractive indices of materials or a period of each material. Furthermore, in the case where the photonic crystal has a photonic bandgap in all the directions, it is easy to realize optical devices with favorable performance, and application fields of the photonic crystal may be broadened as well.
Most studies thus far have focused on one-dimensional or two-dimensional photonic crystals because they can be easily fabricated. However, since one-dimensional or two-dimensional photonic crystals cannot have a photonic bandgap in a specific direction, they cannot act as a photonic crystal in the specific direction. Thus, in consideration of the efficiency and functionality of optical devices employing photonic crystals, there is required a three-dimensional photonic crystal having a periodicity in all the directions. The three-dimensional photonic crystal with complete bandgap for omnidirectional propagation of electromagnetic waves may be variously applied as optical devices to fiber-based telecommunications applications, single mode waveguides, channel add-drop filters, catalysis, and controllers of spontaneous emission.
Typically, manufacturing methods of such three-dimensional photonic crystals can largely be classified into self-assembly and etching methods.
In the self-assembly method, a three-dimensional structure is prepared using colloid or block copolymer as a unit cell and a refractive index of that structure or material is modified, thus obtaining the photonic crystal. To this end, a colloidal templating method has been suggested, which is fully described in U.S. Patent Publication No. 2004/0053009 A1, published on Mar. 18, 2004. The colloidal templating method includes: 1) synthesizing and crystallizing uniform silica or polystyrene particles to create a colloidal crystal; 2) filling an inorganic material such as silica and titania (TiO2) into interstitial voids between the particles to thereby prepare a complex; and 3) removing the silica or polystyrene particles using a solvent or a heat-treatment to form an inverse opal structure. The process of filling the inorganic material into the interstitial voids may be performed by sol-gel, chemical vapor deposition (CVD), simultaneous ordering of a slurry, and so forth.
In the etching method, a predetermined portion is formed through a photoexposure process, and an unnecessary portion is then removed through an etching process. This method, however, is basically adapted to fabricate a two-dimensional pattern, and hence a complicated process must be performed to obtain a three-dimensional photonic crystal. For example, the etching method may be performed using layer-by-layer, wafer fusion, two-photon polymerization, electron beam lithography, glancing-angle deposition, holography or multiple-beam interference methods.
In particular, as in FIG. 1, the layer-by-layer method forms a layer-by-layer structure in which rods of the same layer are parallel with one another and alternately arranged perpendicularly to those of neighboring layers. Specifically, the layer-by-layer method includes: arranging rods at a predetermined regular interval, i.e., “a”⊚(not shown), to thereby form a first layer; forming a second layer in the same manner except that the second layer is rotated 90° with respect to the first layer; forming a third layer parallel to the first layer; and forming a fourth layer parallel to the second layer. Here, the third layer is horizontally shifted by half (a/2) the interval with respect to the first layer, and likewise, the fourth layer is horizontally shifted by half (a/2) the interval with respect to the second layer. A difference in refractive indices or dielectric constants between the rods and air surrounding them can form a photonic crystal. These four layers can constitute a unit cell of the layer-by-layer photonic structure. Therefore, it is possible to form a large photonic crystal in which the unit cells are periodically arranged, when repeating the same procedure of forming the four layers as described above.
The rods are mainly formed of polymer such as polyurethane. Hence, the layer-by-layer photonic crystal cannot exhibit a three-dimensional photonic bandgap because the polyurethane has a low refractive index of about 1.5.
To improve a three-dimensional photonic bandgap, there has been proposed an alternative method in which a high-refractive-index ceramic material, e.g., TiO2 or the like, is infiltrated into a rod type layer-by-layer structure, and heat-treatment is then performed at a high temperature to remove the polymer material. In this case, however, the thickness of a thin film is reduced while the polymer is volatilized during the heat-treatment, leading to the deformation of a thin film.